Extraterrestrial beings monitoring Earth might view changes in our atmosphere as symptomatic of a a self-destructing civilization and take drastic action to keep us from becoming a more serious threat, according to a highly speculative scenario developed last year by scientists at NASA and Penn State University.

Shawn Domagal-Goldman of NASA's Planetary Science Division and his colleagues developed scenarios that could unfold in the aftermath of a close encounter, to help humanity "prepare for actual contact".

Their report, Would Contact with Extraterrestrials Benefit or Harm Humanity? A Scenario Analysis, divides alien contacts into three broad categories: beneficial, neutral or harmful.

Beneficial encounters ranged from the mere detection of extraterrestrial intelligence (ETI), for example through the interception of alien broadcasts, to contact with cooperative organisms that help us advance our knowledge and solve global problems such as hunger, poverty and disease.

Another beneficial outcome the authors entertain sees humanity triumph over a more powerful alien aggressor, or even being saved by a second group of ETs. "In these scenarios, humanity benefits not only from the major moral victory of having defeated a daunting rival, but also from the opportunity to reverse-engineer ETI technology," the authors write.

Other kinds of close encounter may be less rewarding and leave much of human society feeling indifferent towards alien life. The extraterrestrials may be too different from us to communicate with usefully. They might invite humanity to join the "Galactic Club" only for the entry requirements to be too bureaucratic and tedious for humans to bother with.

The most unappealing outcomes would arise if extraterrestrials caused harm to humanity, even if by accident. While aliens may arrive to eat, enslave or attack us, the report adds that people might also suffer from being physically crushed or by contracting diseases carried by the visitors. In especially unfortunate incidents, humanity could be wiped out when a more advanced civilisation accidentally unleashes an unfriendly artificial intelligence, or performs a catastrophic physics experiment that renders a portion of the galaxy uninhabitable.

To bolster humanity's chances of survival, the researchers call for caution in sending signals into space, and in particular warn against broadcasting information about our biological make-up, which could be used to manufacture weapons that target humans. Instead, any contact with ETs should be limited to mathematical discourse "until we have a better idea of the type of ETI we are dealing with."

The authors warn that extraterrestrials may be wary of civilisations that expand very rapidly, as these may be prone to destroy other life as they grow, just as humans have pushed species to extinction on Earth. In the most extreme scenario, aliens might choose to destroy humanity to protect other civilisations.

"A preemptive strike would be particularly likely in the early phases of our expansion because a civilisation may become increasingly difficult to destroy as it continues to expand. Humanity may just now be entering the period in which its rapid civilisational expansion could be detected by an ETI because our expansion is changing the composition of the Earth's atmosphere, via greenhouse gas emissions," the report states.

"Green" aliens might object to the environmental damage humans have caused on Earth and wipe us out to save the planet. "These scenarios give us reason to limit our growth and reduce our impact on global ecosystems. It would be particularly important for us to limit our emissions of greenhouse gases, since atmospheric composition can be observed from other planets," the authors write.

Even if we never make contact with extraterrestrials, the report argues that considering the potential scenarios may help to plot the future path of human civilisation, avoid collapse and achieve long-term survival.

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These scenarios seem trivial in scope and concept as if we are planning for World War I. Yes, WW-01. ET visitors would have to have far-far more advanced technologies than us for them to show up here. We could not hope to combat them, our usual reaction to the unusual, and we don’t yet have the species sophistication to comprehend what they might have to offer or require.

Physicists like to use a phrase, which I like to quote from time to time: "non-zero probability." It essentially means that, while the proposed event is not truly impossible, one would likely have to wait longer than the life expectancy of the universe to see it happen.

Most of these scenarios -- particularly the negative ones -- have roughly that kind of "non-zero probability."

The extraterrestrial disease scenario is particularly odd, at least to me. If their diseases could wipe us out, then ours could do the same to them. If they're that much more advanced than us, they'd be aware of both possibilities and be smart enough to take appropriate precautions.

As for the "crazy green aliens" scenario, I think it's worth considering that whatever steps they might take to wipe us out could do more damage to the ecosystem than leaving us be. (I'm not saying that that's actually the case, mind you; only that it's a possibility worth considering.)

If any of these scenarios happen it will almost certainly be because we were contacted by some radical group (the "crazy green aliens" scenario, or one like it, seems the most likely for this) who ignored the usual laws, precautions, and cultural values of their people.

And people at NASA are worried that our greenhouse gas emissions and radio-based broadcasts are sending a signal that we're dangerous and destructive? Even if that's true (and I sure can't say that it isn't), those signals are traveling outward at the speed of light, which on a galactic scale is incredibly slow. Anything that ETs detect will, at worst, warrant further investigation to see if anything is being done about it. We do have significant segments of our population trying to not only stop but also repair the damage done by war and environmental carelessness.

If They can monitor our planet and discern what is happening so well, then they also obviously know how much effort it takes to influence anything beyond a planet's atmosphere. They also know that interstellar travel is the pipe dream of all but the most advanced civilizations..... Everything that happens on Earth is due to naturally occurring or human influenced naturally occurring events. Everything that happens on Earth happens in space, too, which our scientists know well. Fear mongering and political agenda-driven is what this is.

"To bolster humanity's chances of survival, the researchers call for caution in sending signals into space, and in particular warn against broadcasting information about our biological make-up, which could be used to manufacture weapons that target humans. Instead, any contact with ETs should be limited to mathematical discourse 'until we have a better idea of the type of ETI we are dealing with.'"

I wouldn't say this is fear mongering or a political agenda. Say you were given a task of coming up with possible scenarios why aliens might arrive on planet earth. This would be a possibility. Not a creative one, but possible. Then it gets picked up by a reporter who also has no creativity and liked the movie who is trying to connect with an audience to create revenue. Job done on all accounts.

Yes . We paid for this and much more . I do not live in the U.S . And have no plans too. But I am invested in some of your companies . So I pay taxes there on my capital gains and dividends . I for one think this so called crap is money well spent on . Anything that broadens your reach and intellect is money well spent. I can and do vote in your system . Not at the ballot box but more importantly in procsy and with my dollars . And so does much of the rest of the world . The u s. dollar is a safe haven for money as we all should know and this so called crap is part of the reason. So keep it coming . If you keep pressing every button you find sooner or later you find the right one .

I guess they (probably) won't take any drastic action because we are not a bigger danger than to ourselves, for now. Some civilizations as soon as they know or suspect ywe have detected them, will take action. But it's something for our descendants to deal with :-)

Personally, I believe we've been lied to for a long long time - there is possible evidence in ancient monolithic sites that suggest we've been visited & even helped, seeded by other species. Follow the money, NASA airbrushed moon & mars photos for gods sake - now why would they airbrush a supposedly dead & lifeless planet? Here's another scenario - what if the aliens know all about us & the elite ... They want to help humanity by removing the military industrial complex. The only chance the elite would have is trying to convince us they were an enemy just like they took away our civil rights with the patriot act telling us "it's for our own protection" pfffft what a joke.

Any civilization has its continued survival as its primary concern, and would not be interested in helping another civilization unless by giving that help would be enhancing its own survival probabilities. Any contact scenario must consider that the aliens value their own survival much higher than they value ours.

With that in mind, it seems a very low order of probability can be assigned to scenarios in which the "Space Brothers" give humans the means to solve the problems of human civilization, especially those that would enable humans to thrive and advance technologically to levels with which the Earthlings may one day be able to visit the alien's home world.

It is remotely possible, however, that such a benign contact could take place if the aliens' intent was to promote human development to the point where they could be used as a proxy force against some other civilization the aliens did regard as a particularly troublesome threat. Such a strategy could even be used to "kill two birds with one stone," if the development is limited to the point where neither the human nor the other civilization could survive their conflict.

At the present time (and the foreseeable future) humans are unlikely to be viewed as anything more than a local infection of the cosmos, presenting a danger only to themselves. If humans can survive into a future in which they do have the means of threatening the health of another civilization, then that civilization (if it is advanced enough to do the job) will find it in their interests to eradicate humanity before the infection can spread. Even if humans prevail, eventually they will pose a perceived threat to someone with the means of "curing" the infection.

Since there is nothing available on Earth that is not equally available elsewhere at far less cost to the aliens (especially when uninhabited sources are considered), with the exception of humans and other life native to Earth, and since humans and other Earthly life does not pose an appreciable threat to anyone or anything outside their home planet, it is difficult to imagine why an alien civilization would go to the trouble of contacting us at all, especially in light of the potential risk that such contact might accidentally give humans the needed "boost" to develop into a future threat.

Most likely, first contact would only occur after it was decided that humans already posed a distinct threat; thus, contact would bring with it the promise of human extinction.

We don't need an alien intelligence to solve all our problems. We can do it all by ourselves. Hunger, war, disease, all can be solved here on earth, by us silly humans. And it's no big stretch either. The only problem we have is silly stupid, greedy, paranoid, useless leaders, not to mention the whole religious thing. In my opinion, aliens, whether they are real or not, should steer clear of earth, we don't need them, nor for that matter are they invited here. Leave us alone. We're uite capable of doing things on our own.